MEMS devices are becoming increasingly popular. MEMS transducers, and especially MEMS capacitive microphones, are increasingly being used in portable electronic devices such as mobile telephone and portable computing devices.
Microphone devices formed using MEMS fabrication processes typically comprise one or more moveable membranes and a static backplate, with a respective electrode deposited on the membrane(s) and backplate, wherein one electrode is used for read-out/drive and the other is used for biasing. A substrate supports at least the membrane(s) and typically the backplate also. In the case of MEMS pressure sensors and microphones the read out is usually accomplished by measuring the capacitance between the membrane and backplate electrodes. In the case of transducers, the device is driven, i.e. biased, by a potential difference provided across the membrane and backplate electrodes.
FIGS. 1 and 2 show a schematic diagram and a perspective view, respectively, of a known capacitive MEMS microphone device 100. The capacitive microphone device 100 comprises a membrane layer 101 which forms a flexible membrane which is free to move in response to pressure differences generated by sound waves. A first electrode 102 is mechanically coupled to the flexible membrane, and together they form a first capacitive plate of the capacitive microphone device. A second electrode 103 is mechanically coupled to a generally rigid structural layer or back-plate 104, which together form a second capacitive plate of the capacitive microphone device. In the example shown in FIG. 1a the second electrode 103 is embedded within the back-plate structure 104.
The capacitive microphone is formed on a substrate 105, for example a silicon wafer which may have upper and lower oxide layers 106, 107 formed thereon. A cavity 108 in the substrate and in any overlying layers (hereinafter referred to as a substrate cavity) is provided below the membrane, and may be formed using a “back-etch” through the substrate 105. The substrate cavity 108 connects to a first cavity 109 located directly below the membrane. These cavities 108 and 109 may collectively provide an acoustic volume thus allowing movement of the membrane in response to an acoustic stimulus. Interposed between the first and second electrodes 102 and 103 is a second cavity 110. A plurality of holes, hereinafter referred to as bleed holes 111, connect the first cavity 109 and the second cavity 110.
A plurality of acoustic holes 112 are arranged in the back-plate 104 so as to allow free movement of air molecules through the back plate, such that the second cavity 110 forms part of an acoustic volume with a space on the other side of the back-plate. The membrane 101 is thus supported between two volumes, one volume comprising cavities 109 and substrate cavity 108 and another volume comprising cavity 110 and any space above the back-plate. These volumes are sized such that the membrane can move in response to the sound waves entering via one of these volumes. Typically the volume through which incident sound waves reach the membrane is termed the “front volume” with the other volume, which may be substantially sealed, being referred to as a “back volume”.
In some applications the backplate may be arranged in the front volume, so that incident sound reaches the membrane via the acoustic holes 112 in the backplate 104. In such a case the substrate cavity 108 may be sized to provide at least a significant part of a suitable back-volume. In other applications, the microphone may be arranged so that sound may be received via the substrate cavity 108 in use, i.e. the substrate cavity forms part of an acoustic channel to the membrane and part of the front volume. In such applications the backplate 104 forms part of the back-volume which is typically enclosed by some other structure, such as a suitable package.
It should also be noted that whilst FIGS. 1 and 2 show the backplate being supported on the opposite side of the membrane to the substrate, arrangements are known where the backplate is formed closest to the substrate with the membrane layer supported above it.
In use, in response to a sound wave corresponding to a pressure wave incident on the microphone, the membrane is deformed slightly from its equilibrium or quiescent position. The distance between the membrane electrode 102 and the backplate electrode 103 is correspondingly altered, giving rise to a change in capacitance between the two electrodes that is subsequently detected by electronic circuitry (not shown).
The membrane layer and thus the flexible membrane of a MEMS transducer generally comprises a thin layer of a dielectric material—such as a layer of crystalline or polycrystalline material. The membrane layer may, in practice, be formed by several layers of material which are deposited in successive steps. Thus, the flexible membrane 101 may, for example, be formed from silicon nitride Si3N4 or polysilicon. Crystalline and polycrystalline materials have high strength and low plastic deformation, both of which are highly desirable in the construction of a membrane.
The backplate layer may also be formed of a dielectric material and may be conveniently formed of the same material as the membrane layer e.g. silicon nitride. The backplate supports a backplate electrode and acts as a fixed reference against which the displacement of the membrane and membrane electrode varies. Therefore, the backplate should be rigid and so is typically formed of a thicker layer of dielectric material than the membrane.
The membrane electrode 102 of a MEMS transducer is typically a thin layer of metal, e.g. aluminium, which is typically located in the centre of the flexible membrane 101, i.e. that part of the membrane which displaces the most. It will be appreciated by those skilled in the art that the membrane electrode may be formed by depositing a metal alloy such as aluminium-silicon for example. The membrane electrode may typically cover, for example, around 40% of area of the membrane, usually in the central region of the membrane.
The backplate electrode—which is typically a thin layer of metal e.g. aluminium—is usually embedded within the backplate structure. Thus, the backplate may be formed of a plurality of backplate layers wherein a metal layer which forms the backplate electrode is sandwiched between two adjacent layers.
FIG. 3 shows a simplified cross-sectional view of a conventional MEMS device such as that shown in FIGS. 1 and 2. The backplate 4 comprises a raised portion 4a—which extends in a plane overlying the upper surface of the substrate 5 will typically comprise the acoustic holes—and a sidewall portion 4b which extends between a plane at or close to an upper surface of the substrate (depending on the particular design and whether the membrane layer extends over the region where the sidewall of the baseplate will otherwise land on the top surface of the substrate) and the plane of the raised portion. As mentioned above, both the back-plate 4 and the membrane 1 may be formed from silicon nitride, for example, and the substrate from silicon. However, the thermal expansion coefficient of silicon is greater than that of silicon nitride and this may lead to stresses at the interface between the two dissimilar materials.
The structure of FIG. 3 is formed by various processes of depositing layers and then selectively dry or wet etching portions of the layers away again. These processes take place at relatively low temperatures (in the order of 10-400° C.). When the layers are deposited, there are no intrinsic stress concentrations in the structure. When the structure is released by removal of the sacrificial layers the tensile stress of the deposited layer causes a torsional moment in the membrane sidewall. This leads to a tensile stress concentration on the outer sidewall edge and a compressive stress concentration on the inner sidewall edge. A similar stress can be found in the membrane 1.
These stress concentrations tend to cause cracking originating at the points labelled A and B in FIG. 3, and can potentially lead to failure of the MEMS device. This stress can also render the MEMS device more susceptible to failure during fabrication. For example, when multiple MEMS devices are fabricated on a single wafer and subsequently separated using a technique known as singulation or dicing, the stress at points A and B can cause the device to crack and fail.
In a previous application by the same Applicant, and as illustrated in FIG. 4a, a MEMS transducer has been proposed in which one or more columns 216 are formed which serve to connect the backplate 204, or the backplate and the membrane 201, to the substrate 205. The columns are typically formed around the periphery of the backplate. As shown in FIG. 4a, the columns are provided in a region inside the sidewall portion 204b of the backplate within a region overlying the substrate 205 (i.e. in a region laterally outside the region overlying the substrate cavity).
Although the provision of columns has proved to be effective at increasing the rigidity of the backplate structure and, thus, alleviating stresses arising e.g. at interfacial surfaces where the sidewall of the backplate makes contact with the substrate (either directly or via one or layers provided on top of the substrate), there is a need to further reduce stresses arising in the backplate structure of a MEMS transducer.
Example embodiments described herein are generally concerned with improving the efficiency and/or performance of a MEMS transducer structure. In particular, example embodiments described herein relate to MEMS transducers and processes which seek to alleviate stresses arising within the backplate structure and/or which seek to enhance the rigidity of backplate structures.